S6C10 - Queuing

Which Packet Gets Processed First

Queuing

• the process the router uses to schedule packets for transmission during periods of congestion. – mission-critical and delay-sensitive traffic can be sent

first

• four methods of queuing: – first in,first out (FIFO) queuing; priority queuing (PQ);

custom queuing (CQ); and weighted fair queuing (WFQ)

• Only one queuing method per interface

FIFO Quequing

• Simplest algorithm for packet transmission. – transmission occurs in the same order as

messages are received– Until recently, FIFO queuing was the default

for all router interfaces

• Mission critical packet can be stuck in line behind massive ftp download

Prioritization

• Most effective on WAN links with combination of bursty traffic and relatively lower data rates

• Most effective when applied to links at T1/E1 bandwidth speeds or lower. – If congestion on the WAN link does not exist, there is

no reason to implement traffic prioritization.

– Effective on WAN links that experience temporary congestion.

– Add bandwidth if congesting is constant

Queuing Policy

• provides an appropriate level of service for all users• control expensive WAN costs

– Determines relative importance of different traffic types

– Determines which queuing scheme is best

– Provides ability to prioritize, reserve, and manage network resources,

– ensures the seamless integration and migration of disparate technologies without unnecessary costs.

Choosing a Cisco IOS Queuing Option

• Determine whether the WAN is congested• Decide whether strict control over traffic

prioritization is necessary and whether automatic configuration is acceptable– study the types of traffic using the interface– decide relative priority

• Establish a queuing policy• Determine whether any of the traffic types you

identified in your traffic pattern analysis can tolerate a delay.

Weighted Fair Queuing• Dynamic queuing strategy

– used by default on serial interfaces at E1 speeds (2.048 Mbps) and below.

– Disabled on serial interfaces that use X.25, Synchronous Data Link Control (SDLC), or compressed Point-to-Point Protocol (PPP).

• Uses a complex algorithm to sort the packets that make up the different conversations on an interface

• Automatically allocates bandwidth to all types of network traffic– prioritizes delay-sensitive packets so that high-volume

conversations don’t consume all of the available bandwidth.

– Low-volume traffic streams (which are the majority of traffic) receive preferential service, transmitting their entire loads in a timely fashion.

WQF Discriminators

• Discrimination of traffic into conversations is based on packet-header addressing.

• Common conversation discriminators include: – Source/destination network address

– Source/destination MAC address

– Source/destination port or socket numbers

– Frame Relay Data Link Connection Identifier (DLCI) value

– Quality of service/type of service (QoS/ToS) value

Configuration Commands

• Router(config-if)#fair-queue {congestive-discard-threshold} – congestive-discard-threshold is the number of

messages to queue for high-volume traffic– Fair-queue command appears in the output only

if the congestive discard threshold is modified to a value other than 64

– Use no fair-queue command to disable WFQ– and enable FIFO queuing on an interface

Priority Queuing

• highest-priority traffic always gets dispatched before any other packets– Assign traffic to one of four output queues: high,

medium, normal, or low priority • Once the high queue is empty, the router checks the medium

queue• the lower-priority queue might not be serviced within an

acceptable time frame, or even at all.• used on low-speed WAN links

– Packet is classified; if the appropriate queue is full, the packet is dropped

Configuring Priority Queuing

• Priority list is a set of rules that describe the waypackets should be assigned to PQs– Router(config)#priority-list list-number

protocol protocol-name {high | medium | normal | low} queue-keyword keyword-value

• Router(config)#priority-list 1 protocol ip high tcp 23

• Router(config)#access-list 10 permit 239.1.1.0 0.0.0.255

• Router(config)#priority-list 1 protocol ip high list 10

Priority Queuing Examples

• Establish queuing priorities on packets entering from a given interface: – Router(config)#priority-list list-number interface interface-

type interface-number {high | medium | normal | low}

• Place traffic from E0 in medium priority– Router(config)#priority-list 2 interface ethernet 0 medium

• Change the number of packets– Router(config)#priority-list list-number queue-limit high-

limit medium-limit normal-limit low-limit • Router(config)#priority-list 4 queue-limit 10 40 60 80

Custom Queuing• Reserves a minimum amount of bandwidth for every kind of

traffic– delay-sensitive and mission-critical traffic can be assigned a

large percentage of available bandwidth,– low-priority traffic receives a smaller portion.

• can configure up to 16 queues• Each queue is serviced sequentially

– until the number of bytes sent exceeds the configurable byte count– or until the queue is empty.

• Important for time-sensitive protocols, such as voice, video, or IBMs SNA– require predictable response time. – Queue 0 is a system queue that handles system packets such

as keepalives. • emptied before the other custom queues.

Custom Queuing

• Traffic filtering - The forwarding application-such as IP, IPX, or AppleTalk-– applies a set of filters or access-list entries to each message

that it forwards. • messages are placed in queues, based on the filtering.

• Queued message forwarding – – CQ uses a round-robin dispatching algorithm to forward

traffic. • Each queue continues to transmit packets until the configured byte

limit is reached. • When the threshold of this queue is reached or the queue is empty,

the queuing software services the next queue in sequence.

Configuring Custom Queuing

• Send all traffic from Ethernet interface 0 to custom queue 1.

• Send all IP traffic to custom queue 2.

• Send all IPX traffic to custom queue 3.

• Send all AppleTalk traffic to custom queue 4

Custom Queuing Examples

• Router(config)#queue-list list-number protocol protocol-name queue-number queue-keyword keyword-value

• Router(config)#queue-list list-number interface interface-type interface-number queue-number

Queuing Show Commands

• show queueing

• show interfaces

• show queueing custom

• show queueing priority

• show queueing fair

Data Compression

• identifies patterns in a stream of data, and chooses a more efficient method of representing the same information– algorithm is applied to the data to remove as

much redundancy as possible• Shannon's Limit

– how much a given source of data can be compressed

Types of Data Compression

• Link compression (also known as per-interface compression)

• Payload compression (also known as per-virtual-circuit compression)

• TCP header compression – By default data is sent uncompressed

Link Compression

• uses either the Predictor or STAC algorithm to compress the traffic

• To ensure error correction and packet sequencing (Cisco High-Level Data Link Control [HDLC] uses STAC compression only):– Predictor - Predicts the next sequence of characters in the

data stream by using an index to look up a sequence in a compression dictionary

– STAC - Developed by STAC Electronics, STAC is a Lempel-Ziv (LZ)-based compression algorithm. It searches the input data stream for redundant strings and replaces them with what is called a token,

Payload Compression

• (also known as per-virtual-circuit compression) – compresses only the data portion (including the Layer 3 and

Layer 4 headers) of the data stream– frame header is left untouched

• appropriate for virtual network services such as Switched Multimegabit Data Service (SMDS), Frame Relay, and Asynchronous Transfer Mode (ATM).

• Use the frame-relay payload-compress command to enable STAC compression on a specified Frame Relay point-to-point interface or subinterface:– Router(config-if)#frame-relay payload-compr

TCP/IP Header Compression

• subscribes to the Van Jacobson Algorithm, which is defined in RFC 1144.– protocol specific and compresses only the TCP/IP

header, which leaves the Layer 2 header intact to allow a packet with a compressed TCP/IP header to travel across a WAN link

– Don’t implement both Layer 2 payload compression and TCP/IP header compression

– header compression is generally used at lower speeds, such as 64-kbps links.

• Router(config-if)#ip tcp header-compression [passive]

Compression Considerations

• Modem compression - In dial environ-ments, compression can occur in the modem.

• Encrypted data - Compression is a Layer 2 function. When a data stream is encrypted by the client application, it is then passed onto the router for routing or compression service

• CPU cycles versus memory - The amount of memory that a router must have varies according to the protocol being compressed